1. Field of the Invention
The invention is directed to a sintered shaped ceramic body formed of Y2O3-stabilized zirconia and to a process for producing a sintered shaped ceramic body formed of Y2O3-stabilized zirconia such as are known generically from the publication by Muñoz-Saldaña J., et al. (2003, Journal of Materials Research 18, 2415-2426).
2. Discussion of Background Information
Zirconia (ZrO2) is a ceramic that is used, for example, as a refractory ceramic, as a technical ceramic and in prosthetics. The crystal structure of zirconia is classified in accordance with the crystal systems commonly used for the domain of crystallography. These crystal systems are listed, for example, in Hahn, T. (1983: International Table for Crystallography, Reidel, Dordrecht).
Tetragonal stabilized zirconia (hereinafter for brevity called TZP) is one of the strongest high-performance ceramics on the market. TZP ceramics are used, for example, in mechanical engineering, as bioceramics and for household articles.
Certain metal oxides such as Y2O3 (yttrium oxide), CaO (calcium oxide), MgO (magnesium oxide) or CeO2 (ceric oxide) are used in small concentrations for doping in order to stabilize the tetragonal phase. The highest strengths are achieved with 3 mol % Y2O3-doped zirconia. For this reason, it is also called 3Y-TZP. The tetragonal phase is metastable in this material, i.e., it is not thermodynamically stable, but a transformation to the monoclinic phase is kinetically inhibited. At room temperature, a phase transition of this type without external influences within finite times does not take place.
The strength of 3Y-TZP and other Y-TZPs derives from what is known as transformation toughening, where the tetragonal phase transforms to the monoclinic phase under the influence of mechanical stresses before a fracture of the material occurs. In so doing, the fracture energy is consumed and compressive stresses are induced in the material. These compressive stresses can counteract further crack propagation. Therefore, the strength of the material depends upon the thermodynamic instability of the tetragonal phase and on the parameters causing the phase transformation.
However, the instability of the tetragonal phase does not only have a positive effect on the behavior of the material. In a moist environment, e.g., in water or in air saturated with water vapor, the phase transformation already takes place spontaneously at temperatures of <100° C. In this case, the tetragonal phase transforms spontaneously into the monoclinic phase. This process is known as “hydrothermal aging” or “low temperature degradation”. Hydrothermal aging depends on the temperature and on the concentration of water molecules in the surrounding atmosphere.
Hydrothermal aging always starts at the surface of the material and progresses at a relatively constant rate into the volume of the material. Initially, areas near the surface are affected so that roughness increases and the hardness of the material decreases. Particularly in components with polished surfaces or in wear couples, enormous economic losses are caused by hydrothermal aging. The life of these components is appreciably reduced. While hydrothermal aging has no noticeable effect on component strength in components with a high volume-to-surface ratio, it results in catastrophic failures particularly for components having a surface which performs a specific function.
In dental restorations comprising 3Y-TZP, the translucency of the material plays an important role for aesthetic reasons. Therefore, efforts are made to produce crown frameworks and bridge frameworks of 3Y-TZP, for example, with high translucency as well as high hydrothermal resistance. Heretofore, these requirements could be reconciled only to a limited extent.
Over the past decade, considerable efforts have been made to increase the hydrothermal stability of 3Y-TZP. This can be accomplished in principle by an increase in the amount of stabilizer contained in the material; a homogeneous distribution of the stabilizer in the matrix of the material, by reducing the grain size, and by the addition of Al2O3 of ≧0.25 mass %. The increased stabilizer content leads to a deterioration of the mechanical properties. The addition of Al2O3 according to manufacturer's specifications leads to increased hydrothermal stability, but the translucency of the ceramic is substantially reduced because of scattering phenomena.
It is known that the tetragonal phase can be stabilized when the grain size is below a critical value. The critical grain size is specified at 360 nm in Muñoz-Saldaña et al. (2003), at 370 nm in Chen et al. (1989: Journal of Materials Science 24(2): 453-456) and at 400 nm in Tsukuma et al. (1984: Science and technology of zirconia 11: 382-390). Other authors report that the TZP ceramic prepared by them with a grain size of 200 nm was not aging-stable (Kern et al, 2011: Journal of Ceramic Science and Technology 2(3): 147-154).
The publication by Suárez et al. (2009: Science and Technology of Advanced Materials 10(2): 25004) discloses sintered shaped bodies with mean grain sizes of about 100 nm which are exclusively in tetragonal phase and were produced from nanopowders with particle sizes of 65 nm.
As defined by the EU Commission of Oct. 18, 2011, nanomaterials are materials with particle sizes of less than 100 nm (e.g., nanoparticles, nanoplates or nanofibers).
The preparation of an aging-stabilized 3Y-TZP comprising commercial nanopowders with a particle size of the nanopowder of 65 nm is known from WO 2010/061196 A2.
The known processes have the drawback that they use expensive nanopowders making it impossible to produce aging-stable sintered shaped bodies inexpensively.
The use of 50 μm size grinding media for deagglomeration of nanopowders is known from the publications by Suarez et al., (Suarez et al., 2010, Journal of Nanoscience and Nanotechnology 10: 6634-6640, and Suarez et al., 2009, Science and Technology of advanced Materials 10, doi: 10/1088/1468-6996/10/2/025004). Agglomerates are formed when nanopowder is stored over extended periods of time. The tendency of nanopowders to form such agglomerates is another drawback to their use.
The transmission of visible light is sharply reduced in Y-TZPs because of the birefringence of tetragonal zirconia. According to Klimke et al. (2011: Journal of the American Ceramic Society 94 (6), 1850-1858), a theoretical inline transmission of about 7% can be measured at a wavelength of 550 nm by reducing the grain size to 150 nm. The human eye has the greatest sensitivity at this wavelength.
Muñoz-Saldaña et al. have shown that an aging-stable 3Y-TZP ceramic with a mean grain size of 320 nm and a three point bending strength of 1400 MPa can be realized with commercial submicron powder.